import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { Observable, of } from 'rxjs';

@Injectable()
export class ClientSideCacheService {
	/**
	 * Cached values are stored in sessionStorage in the form {key:value}
	 * Cached values are available to the users of the service in the form of observables
	 */

	// Need to read all cachedValues into memory here
	constructor(private http: HttpClient) {
	}

	static readonly CACHE_TIME: number = 10; // Default cache time in minutes

	/**
	 *
	 * @param key The key of the desired
	 * @param url Specifies the url where the value can be found if it is not present in the cache
	 * @param timeToLive Cache invalidation time in minutes - only used if cache is stale
	 */
	public getValue(key: string, url: string, timeToLive?: number): Observable<any> {

		// Check if we have the value cached
		const value = JSON.parse(sessionStorage.getItem(key));
		let obsValue: Observable<any> = new Observable<any>();

		if (value) {
			obsValue = of(value);
		}

		// Read value if it is not yet read or has been invalidated
		// (this works, because no value = less than now)
		const isStale = new Date(sessionStorage.getItem(`${key}-invalidation-time`)) < new Date();
		if (isStale) {
			if (value) {
				this.http.get(url).subscribe( data => {
					if (data) {
						this.setValue(key, data, timeToLive);
					}
				});
			} else {
				obsValue = new Observable((observer) => {
					this.http.get(url).subscribe( data => {
						observer.next(data);
						observer.complete();
						this.setValue(key, data, timeToLive);
					});
				});
			}
		}

		return obsValue;
	}

	/**
	 * @param key The key that can later be used to get the value
	 * @param value The value to be cached. Can later be retrived by using the key parameter
	 * @param timeToLive Cache invalidation time in minutes
	 */
	public setValue(key: string, value: any, timeToLive?: number): void {
		sessionStorage.setItem(key, JSON.stringify(value));
		sessionStorage.setItem(
			`${key}-invalidation-time`,
			new Date(new Date().getTime() + ((timeToLive !== undefined)? timeToLive  : ClientSideCacheService.CACHE_TIME) * 60 * 1000).toString());
	}

}